Support optional project context in skill suggestion (#299)

[DaniloTatti]

Summary

Closes part of SprintStartProject/Wiki#299 (Skill suggestion integration).

Relaxes the projectId requirement on POST /api/v1/skills/suggest so roles without project scoping (person-level / global roles) can request AI skill suggestions.

Changes

• Schema (src/api/schemas.py): SkillSuggestionRequest now inherits BaseModel directly and makes project_id: ProjectId | None = Field(default=None, alias="projectId") optional.
• Domain logic (src/skills/suggestion.py):
  • suggest_skills accepts project_id: str | None = None.
  • Guarded RAG retrieval: hybrid_retrieve only runs when project_id is not None and store.count() > 0. When project_id is absent, retrieval is skipped (preserving fail-closed scoping against cross-project retrieval).
  • Grounding gate behavior: without retrieved chunks, project-specific skills citing unretrieved chunks are dropped as ungrounded, while catalog/role-based universal skills pass through cleanly.
• Tests:
  • tests/skills/test_suggestion.py: asserts retrieval is skipped and zero store queries happen when project_id=None.
  • tests/api/test_skills.py: end-to-end API test verifying POST /api/v1/skills/suggest succeeds with 200 without projectId and returns universal suggestions.